French patent application No. 86 14740 (corresponding to U.S. Pat. No. 4,925,811) describes a method of producing a multi-wavelength semiconductor laser which is produced on the basis of a double heterostructure stack composed of alternate confinement layers and active layers of different compositions.
It will be evident to those skilled in the art on the one hand that different wavelengths can be obtained from such a stack by making P-N junctions in the vicinity of active layers on a different rank in the stack. On the other hand, this prior art method is in particular aimed at using only one single multi-layer epitaxy (production of double heterostructures at one go), whatever the number of wavelengths desired. Also, in order to meet this target while producing a multi-wavelength laser, the method according to the prior document envisages, prior to formation of P-N junctions, selective erosion of the top of the stack on parallel and mutually adjacent bands in order to expose confinement layers of a different rank at least on two adjacent bands. The confinement layers which are thus laid bare then define parallel terraces separated from one another by transitions and, at least in two adjacent terraces, the active layers subjacent to the confinement layer above are of a different rank and can easily be reached from the surface of the terraces.
The formation of different P-N junctions is carried out therefore by the introduction of impurities in a band included in each terrace, through a staged contact layer arranged on the free surface of the stack.
This impurity introduction stage can be carried out, for instance, by localised diffusion. For this, a diffusion mask is envisaged which has the same number of slots as the desired number of P-N junctions. The mask is applied to the surface of the stack so that each slot is situated over a terrace and the impurities are diffused through these slots. It will be understood therefore that the staged profile of the stack may create problems with regard to satisfactory application of the mask. Indeed, if this latter is correctly applied to the highest terrace, the stepped profile produces a space between the mask and the subsequent terraces. This space, of course, increases with the number of terraces. This has two disadvantages.
The first relates to the fact that a mask does not properly "stick" to a surface which has any irregularities of shape. This is the case here by virtue of the unevenness of the free surface of the stack.
The second relates to the accuracy of location of the impurities in the terrace. Indeed, the greater the space between the slot in the mask and the upper surface of the terrace, the greater is the diffraction phenomenon through the slot and the less accurate will be the implantation of the impurities. This may lead to a less satisfactory quality of the P-N junction formed and to a lack of accuracy in its positioning.
The invention provides a more satisfactory solution to this problem.